Halobutyl rubbers, which are halogenated isobutylene/isoprene copolymers, are the polymers of choice for best air retention in tires for passenger, truck, bus and aircraft vehicles. Bromobutyl rubber, chlorobutyl rubber and halogenated star-branched butyl rubbers can be formulated for specific tire applications, such as tubes or innerliners. The selection of ingredients and additives for the final commercial formulation depends upon the balance of properties desired, namely, processability and tack of the green (uncured) compound in the tire plant versus the in-service performance of the cured tire composite. Examples of these elastomers are butyl (isobutylene-isoprene rubber or IIR), bromobutyl (brominated isobutylene-isoprene rubber or BIIR), chlorobutyl (chlorinated isobutylene-isoprene rubber or CIIR), star-branched butyl (SBB), EXXPRO™ elastomers (brominated isobutylene-co-p-methyl-styrene copolymer or BIMSM), etc. The present application focuses on processability of halogenated isoolefin polymers, including BIMSM.
It is known to form conventional and nanocomposite tire innerliners using brominated copolymers of isobutylene and para-methylstyrene, and blends of these copolymers with other polymers. See, for example, Elspass et al., U.S. Pat. Nos. 5,807,629, and 6,034,164. Conventional tire innerliners are typically filled with carbon black or another filler, whereas nanocomposites typically can also include clay.
Carbon black is a conventional reinforcing material used in halogenated isoolefin rubbers. The major carbon black used in tire innerliners is N660, which has a nitrogen surface area of 35 m2/g. W. Barbin et al., Chapter 9 in Science and Technology of Rubber, J. E. Mark et al. Eds., 2nd Ed., Academic Press: New York, (1994). N234 is another common carbon black, which has a nitrogen surface area of 126 m2/g and greater reinforcing characteristics. The tack behavior of butyl polymer containing a small amount of tackifier is known from, for example, M. F. Tse, “Green Tack of Butyl Polymers”, Polym. Prepr., vol. 45, no. 1, p. 980 (2004).
Nanocomposites are polymer systems containing inorganic particles with at least one dimension in the nanometer range. Some examples of these are disclosed in U.S. Pat. Nos. 6,060,549, 6,103,817, 6,034,164, 5,973,053, 5,936,023, 5,883,173, 5,807,629, 5,665,183, 5,576,373, and 5,576,372. Common types of inorganic particles used in nanocomposites are phyllosilicates, an inorganic substance from the general class of so called “nano-clays” or “clays.” Due to the general enhancement in air barrier qualities of various polymer blends when clays are present, there is a desire for a nanocomposite with low air permeability; especially a dynamically vulcanized elastomer nanocomposite such as used in the manufacture of tires.
Organoclays are typically produced through solution based ion-exchange reactions that replace sodium ions that exist on the surface of sodium montmorillonite with organic molecules such as alkyl or aryl ammonium compounds and typically known in the industry as swelling or exfoliating agents. See, e.g., U.S. Pat. No. 5,807,629, WO 02/100935, and WO 02/100936. Other background references include U.S. Pat. Nos. 5,576,373, 5,665,183, 5,807,629, 5,936,023, 6,121,361, WO 94/22680, WO 01/85831, and WO 04/058874. Elastomeric nanocomposite innerliners and innertubes have also been formed using a complexing agent and a rubber, where the agent is a reactive rubber having positively charged groups and a layered silicate uniformly dispersed therein. See, for example, Kresge et al. U.S. Pat. Nos. 5,665,183 and 5,576,373.
Regardless of the filler employed, brominated copolymers of isobutylene and para-methylstyrene, and blends thereof used in tire innerliners, would desirably have a processability similar to that of conventional bromobutyl rubber, especially when filled with various levels of carbon black, clay particles, or the like. When a polymer is deformed, a stress builds up in the polymer due to a decrease in entropy. However, even if the polymer is kept in the strained state, the stress will drop or relax because the polymer chains tend to diffuse back to the isotropic state of highest thermodynamic probability. Good processability requires fast stress relaxation of the green or uncured composition when strained to a prescribed deformation. Therefore, as used herein, the terminology of processability and stress relaxation is used synonymously. Poor processability or a slow stress decay or relaxation poses problems in tire building because no operator wants to handle a piece of rubber compound that continues to shrink as time goes on. Although this application has existed for many decades, there are not many systematic studies on the processability of green elastomers loaded with carbon black or other fillers. Usually, the concentration of carbon black ranges from 40-100 phr, where phr stands for parts per hundred of rubber (if elastomer=100 g, then carbon black=40-100 g). Of course, besides processability, it is desired to maintain other elastomer innerliner compound performance advantages as much as possible, such as impermeability, flex fatigue resistance, cured adhesion, etc.
At low carbon black loading in a butyl rubber, the composite can be described as showing a liquid-like behavior. With increasing carbon black loading in many butyl rubbers, a gel-like behavior can occur when the filler has a high enough concentration and/or the polymer has strong enough interactions with the filler so that the filler particles begin to percolate through the polymer to form a continuous network. The carbon black or other filler loading concentration at which gel-like or pseudo-solid-like behavior occurs is referred to as the percolation threshold.
Usually, lower critical filler concentrations or percolation thresholds are the result of stronger polymer/filler interactions, as described in Y. Yurekli et al., “Structure and Dynamics of Carbon Black-Filled Elastomers,” J. Polym. Sci., Polym. Phys. Ed., vol. 39, p. 256 (2001); and M. F. Tse et al., “Structure and Dynamics of Carbon Black Filled Elastomers II, IMS and IR,” Rubber World, vol. 228, no. 1, p. 30 (2003). With increasing filler loadings, the percolation threshold can manifest itself in various ways, for example: a sharp increase in relaxation time; a sharp increase in the area under the stress relaxation curve, referred to as the steady state viscosity (see Strobl, The Physics of Polymers, 2nd Ed., Springer, Germany (1997) (a faster relaxation will result in a smaller area, hence, a lower steady state viscosity or an improvement in processability); an increase in storage modulus (G′) in the low frequency region to a similar magnitude as the loss modulus (G″); etc. In any case, it is observed that processability declines significantly when the loading exceeds the percolation threshold. An explanation of stress relaxation of a polymer is given in W. Tobolsky, Properties and Structure of Polymers, John Wiley & Sons, Inc., New York, N.Y., p. 219 (1960).
The improvement of processability of rubber compounds based on star-branched butyl and halobutyl polymers is known, for example, from Powers et al., U.S. Pat. No. 5,071,913. Unique polymerization conditions enable broadening of the molecular weight distribution via a high degree of branching so that the polymer consists of low molecular weight linear chains that are blended with a small fraction of star-shaped molecules. Processability benefits include faster stress relaxation, enhanced green strength, and improved mixing, calendering and extrusion.
The present invention discloses halogenated isoolefin elastomers filled with carbon black or another filler, having a molecular weight and composition for fast large strain induced stress relaxation of the green rubber compounds, and improved small-strain viscoelastic properties, which determine at what concentration the filler begins to percolate and form a continuous network. It has quite surprisingly been found that, when filled with carbon black, clay or another filler, halogenated isoolefin elastomers, such as, for example, brominated isobutylene-para-methylstyrene elastomers having specified characteristics of alkylstyrene content, bromine content, Mooney viscosity, molecular weight, and branching index, have a similar degree of stress relaxation compared to conventional bromobutyl rubbers.